In the case of field devices having a field device electronics and conductive or capacitive sensor units, it is necessary in the case of extreme field conditions to conduct a calibration of the sensitivity of the field device using a potentiometer, wherein especially a switching threshold of an evaluation electronics is adjusted using the potentiometer. The direction in which the switching threshold is changed depends on whether the field device is being operated in a “MIN” type of operation as pump protection, i.e. warning when a predetermined fill level is subceeded, i.e. fallen below, or in a “MAX” type of operation as overflow protection, i.e. warning when a predetermined fill level is exceeded. The calibration is then always performed, when an indicated sensor state, due to field conditions (accretion, etc.), does not agree with the actual sensor state.
In the case of practically all previously marketed sensor units for fill level determination in liquids and bulk goods, such as work on the basis of capacitance measurements or conductivity measurements, sinusoidal electrical alternating voltage signals are used as drive signals for the sensor units. Concurrently, the alternating signals also serve directly as measurement signals. These alternating signals are normally produced by means of an analog oscillator and, for further processing, analog filtered, rectified, and, in the case of limit level switches, compared by means of analog comparators with predetermined threshold values. Microprocessors are, as a rule, only used to linearize, scale, and provide the signals, prepared by means of analog electronics, with time delays, switching hystereses, or inversions.